In cases where, for example, a manual operation is performed on numerically controlled machines such as robots, an operator often enters a dangerous area to carry out his job. In such cases, a pendant with a push-button switch, such as called an enable switch (or deadman switch), is used for obviating an accident associated with contact with any one of operating machines during the execution of his job.
As shown in FIG. 30, a teaching pendant 101 includes a pendant body 103, opposite end portions of which define grip portions 105a, 105b to be held by respective hands, and which has a centrally located liquid crystal display (hereinafter referred to as “LCD”) 107. As viewing a screen of the LCD 107, the operator may properly manipulate, with his thumbs or the like, a plural number of operation keys 109a, 109b and other operation keys arranged along the opposite sides of the LCD 107, thereby teaching data, such as a program, to a robot or controlling the robot.
In this case, the teaching operation or the like to the robot is not enabled by merely manipulating the operation keys 109a, 109b. As shown in FIG. 31, an arrangement is made such that the robot is disabled for program teaching or control unless operation sections 111a, 111b of a push-button switch (not shown) disposed on back sides of the grip portions of the pendant body 103 are manipulated to shift the push-button switch to an ON state before the operation keys 109a, 109b are manipulated.
As shown in FIG. 32, a snap-action type push-button switch, for example, may be used as such a push-button switch disposed at the teaching pendant.
As shown in FIG. 32, the push-button switch 113 includes a switch case 115 having a rectangular shape in plan; a push button 117 depressibly supported by the switch case 115; a normally close contact 119, as an auxiliary contact, and a pair of c-contacts 121a, 121b disposed in the switch case 115; and a pair of switching mechanisms 123a, 123b disposed in the push button 117 and operatively associated with pressing-down on the push button 117 for switching the c-contacts 121a, 121b between an open position and a close position.
The normally close contact 119 includes a movable member 127 disposed within the switch case 115 as projecting toward the push button 117 (upward) and urged upward by a helical spring 125 disposed thereunder; a pair of movable terminals 129a, 129b attached to the movable member 127; and a pair of stationary terminals 131a, 131b adapted to be brought into or out of contact with the corresponding movable terminals 129a, 129b. In an initial state, the movable member 127 is urged upward by the helical spring 125, so that the normally close contact 119 is closed with the movable terminals 129a, 129b thereof contacting the stationary terminals 131a, 131b thereof.
The c-contacts 121a, 121b have a snap action configuration, individually including a movable terminal 133a, 133b; a normally close stationary terminal 135a, 135b and a normally open stationary terminal 137a, 137b disposed above and below the movable terminal 133a, 133b, respectively; an operative member 139a, 139b for moving the movable terminal 133a, 133b between the normally close stationary terminal 135a, 135b and the normally open stationary terminal 137a, 137b; and a helical spring 141a, 141b anchored to the operative member 139a, 139b and to the movable terminal 133a, 133b. When the c-contacts 121a, 121b are in an initial state or in a first OFF state, the movable terminals 133a, 133b are in contact with the normally close stationary terminals 135a, 135b, as spaced away from the normally open stationary terminals 137a, 137b. 
The switching mechanisms 123a, 123b are disposed in an accommodating portion 143 defined in the push button 117. The switching mechanisms 123a, 123b individually include a pressing piece 145a, 145b for pressing the respective operative member 139a, 139b of the c-contact 121a, 121b; a slide block 147a, 147b for depressing the respective pressing piece 145a, 145b as operatively associated with pressing-down on the push button 117; and a pressing shaft 149a, 149b engaged with the respective slide block 147a, 147b. 
The pressing shafts 149a, 149b are formed with a respective pair of flanges 151a, 151b at a respective upper end thereof. The pressing shafts 149a, 149b are formed with a slope 153a, 153b at one of the flange pair 151a, 151b, respectively. The pressing shafts 149a, 149b are of a hollow structure such that helical springs 155a, 155b anchored to an upper inside surface of the accommodating portion 143 are anchored to respective places within the pressing shafts 149a, 149b. 
The slide blocks 147a, 147b are individually formed with a cavity 157a, 157b vertically extended therethrough, whereas the pressing shafts 149a, 149b are inserted through the cavities 157a, 157b, respectively. The slide blocks 147a, 147b are individually formed with a slope 159a, 159b at an inside wall of one end thereof, the slopes 159a, 159b engaged with the corresponding slopes 153a, 153b of the pressing shafts 149a, 149b. Helical springs 161a, 161b are disposed in individual spaces between the one end of the slide block 147a and a side wall of the accommodating portion 143 and between the one end of the slide block 147b and the other side wall of the accommodating portion 143. The helical springs 161a, 161b urge the slide blocks 147a, 147b toward the center of the push button 117. The respective other end of the slide blocks 147a, 147b is adapted to be pressed against a respective upper end of the pressing pieces 145a, 145b. 
When the push button 117 of the push-button switch 113 in the initial state shown in FIG. 32 or in the first OFF state is depressed, the pressing pieces 145a, 145b push down the operative members 139a, 139b of the c-contacts 121a, 121b as operatively associated with pressing-down on the push button 117, thereby moving the movable terminals 133a, 133b away from the normally close stationary terminals 135a, 135b and into contact with the normally open stationary terminals 137a, 137b, as shown in FIG. 33. Thus, the push-button switch 113 is shifted to an ON state.
When the push button 117 of the push-button switch 113 in the ON state is further depressed, the slopes 159a, 159b of the slide blocks 147a, 147b slide on the slopes 153a, 153b of the pressing shafts 149a, 149b, so that the slide blocks 147a, 147b are moved outwardly relative to the pushbutton 117, as shown in FIG. 34. The movements of the slide blocks 147a, 147b release the individual other ends thereof from the engagement with the individual upper ends of the pressing pieces 145a, 145b which, in turn, are allowed to move upwardly to release the pressure on the operative members 139a, 139b. This causes the movable terminals 133a, 133b to move away from the normally open stationary terminals 137a, 137b and to come into contact with the normally close stationary terminals 135a, 135b. Thus, the push-button switch 113 in the ON state is shifted to a second OFF state.
That is, the push-button switch 113 permits key entry via the teaching pendant 101 only when the two c-contacts 121a, 121b are in the ON state. Furthermore, the push-button switch 113 can be set to any of three positions (the first OFF state, the ON state and the second OFF state) according to the amounts of depression of the push button 117. Hence, the push-button switch provides for a tangible indication of intent of the operator manually operating the machines, thus ensuring the safety of the operator.
Next, an example of circuit connections of the teaching pendant having the above arrangement will be described with reference to FIG. 35.
As shown in FIG. 35, the teaching pendant comprises three circuits which include a first, a second and a third circuits. In the first circuit, one R1 of c-contacts of a right-hand push-button switch and one L1 of c-contacts of a left-hand push-button switch are connected in parallel. In the second circuit, the other c-contact R2 of the right-hand push-button switch and the other c-contact L2 of the left-hand push-button switch are connected in parallel. In the third circuit, a normally close contact R3 of the right-hand push-button switch and a normally close contact L3 of the left-hand push-button switch are connected in series.
Based on such circuit connections, the teaching pendant 101 is enabled for data entry by way of either the right or left hand manipulating the push-button switch for driving the first and second circuits into conduction. Therefore, even in a case where the right hand is fatigued by holding the operation section depressed in order to enable the program teaching, for example, the teaching pendant may be kept enabled for teaching by the left hand, in turn, which depresses the operation section. In addition, the machines can be deactivated by either the right or left hand manipulating the push-button switch to shift the push-button switch to the second OFF state. This leads to an enhanced safety of the operator who is carrying out his job.
It is noted here that NC1-R and NC2-R are equivalent to the normally close stationary terminals 135a, 135b of the c-contacts R1, R2 (the same as the c-contacts 121a, 121b shown in FIG. 32, for example) constituting the right-hand push-button switch; that NO1-R, NO2-R are equivalent to the normally open stationary terminals 137a, 137b of the c-contacts 121a, 121b constituting the right-hand push-button switch; and that C1-R and C2-R are equivalent to the movable terminals 133a, 133b of the c-contacts R1, R2 constituting the right-hand push-button switch.
Likewise, NC1-L, NC2-L are equivalent to the normally close stationary terminals 135a, 135b of the c-contacts L1, L2 (the same as the c-contacts 121a, 121b shown in FIG. 32, for example) constituting the left-hand push-button switch; NO1-L, NO2-L are equivalent to the normally open stationary terminals 137a, 137b of the c-contacts L1, L2 constituting the left-hand push-button switch; and C1-L and C2-L are equivalent to the movable terminals 133a, 133b of the c-contacts L1, L2 constituting the left-hand push-button switch. On the other hand, R3 and L3 are each equivalent to the normally close contact 119 constituting the right-hand or left-hand push-button switch.
In a case where the operator is aware of some potential danger while performing the teaching operation via the teaching pendant with the left-hand push-button switch held in the ON state, for example, it is also possible to deactivate the machines by driving the third circuit into cutoff, as shown in FIG. 36, which is established by depressing the right-hand push-button switch to shift the same to the second OFF state.
In this case, if the pressure on the right-hand push-button switch is released to return the same to the first OFF state while the left-hand push-button switch is maintained in the ON state, as shown in FIG. 37, the first, second and third circuits are brought into conduction again so that the left-hand push-button switch in the ON state is active. Therefore, despite the right-hand push-button switch shifted to the second OFF state as emergency measure, the machines are so quickly enabled for teaching before both the push-button switches are returned to the initial or first OFF state. Consequently, the teaching pendant suffers poor reliability.
It is an object of the present invention to provide an enabler for teaching pendant which ensures that once either the right-hand or the left-hand push-button switch is shifted to the second OFF state, the teaching pendant is never enabled before both the push-button switches are shifted to the first OFF state.